1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for controlling an air-fuel ratio of an internal combustion engine, and particularly to an air-fuel ratio control apparatus in which air-fuel ratio control immediately after start-up of an internal combustion engine is more suitably performed than a conventional one.
2. Description of the Related Art
As is well known, in an internal combustion engine, for the purpose of improving fuel economy and purifying exhaust gas, the so-called air-fuel ratio feedback control (hereinafter also referred to as air-fuel ratio control) is performed. In the air-fuel ratio feedback control, in general, when an air-fuel ratio sensor mounted in an exhaust passage detects that the air-fuel ratio is in a rich state, a fuel injection amount from an injector (injection valve) is reduced to shift the air-fuel ratio to the lean side. Besides, when the air-fuel ratio sensor detects that the air-fuel ratio is in a lean state, the fuel injection amount from the injection valve is increased to shift the air-fuel ratio to the rich side. By performing the control as state above, the control to cause the air-fuel ratio of the gas passing through the exhaust passage of the internal combustion engine to coincide with the target air-fuel ratio is performed.
Immediately after the internal combustion engine is started, since a state of each portion in the inside of the internal combustion engine is different from a normal condition (for example, temperature is low), when the air-fuel ratio control which is set to be optimally operated in the normal condition is used as it is, there is a possibility that various problems arise. For example, patent document 1 (JP-A-8-312428) discloses a technique in which at the time point of start of the air-fuel ratio feedback control immediately after start-up, an integration constant used for the first integration control toward the lean direction is made larger than a normal value, and the control speed is increased to enhance convergence to the target air-fuel ratio, and further, spark advance control for ignition timing is performed in order to suppress a drop in engine rotation number(rotation speed) which occurs since the air-fuel ratio control is not normally performed at the time of start-up (there are also other causes).
However, in the technique disclosed in patent document 1, especially at the time of cold engine start-up (at the time of start-up from the state where the engine is cold), there is a problem that a good control state can not be immediately obtained. For facilitating the understanding of this, a description will be made while the state immediately after the start-up is schematically shown in FIGS. 9A to 9C. In FIGS. 9A to 9C, the horizontal axis indicates the elapsed time after the start-up, and FIG. 9A shows an air-fuel ratio feedback correction coefficient. Besides, FIG. 9B shows an air-fuel ratio with the passage of time. FIG. 9C shows a change of engine rotation speed. In order to certainly perform the cold start-up, a large amount of fuel must be injected at the time of the start-up. Immediately after the start-up, especially immediately after the cold engine start-up, a temporarily very rich air-fuel ratio is produced by the large amount of fuel injected at the time of the start-up (T1 of FIGS. 9A to 9C). Thus, even if the integration constant of the air-fuel ratio feedback control is made larger than a normal value and the control speed is increased (FIG. 9A), the shift of the air-fuel ratio to the target air-fuel ratio is delayed due to a delay time such as a response delay time of a sensor to detect the air-fuel ratio and a transport delay time of fuel injection amount (till T2 of FIGS. 9A to 9C), and the shift to the target air-fuel ratio does not immediately start. Besides, when the shift of the air-fuel ratio toward the lean direction starts and exceeds the target air-fuel ratio, the air-fuel ratio feedback control toward the rich direction is performed this time. However, as described above, since there is the delay time, the air-fuel ratio does not immediately shift toward the rich direction (T2 to T3 of FIGS. 9A to 9C), and during the delay time, the air-fuel ratio is shifted toward the lean direction excessively since the integration constant used for the first integration control toward the lean direction is made larger than the normal value, that is, the control speed toward the lean direction is increased. Such a state is called overshoot (FIG. 9B). There has been a problem that when the overshoot occurs, the rotation speed is reduced, and a misfire (engine stop) finally occurs.
Further, even if the misfire does not occur, in the air-fuel ratio feedback control, in order to return the overshoot state to the target air-fuel ratio, the air-fuel ratio feedback control is performed significantly toward the rich direction. Thus, there has also been a problem that the air-fuel ratio is put in a hunting state with respect to the target air-fuel ratio, and the convergence to the target air-fuel ratio becomes slow (after T4 of FIGS. 9A to 9C)
Besides, when consideration is given to the existence of the fluctuation in characteristics of commercially available fuel among fuel companies and the fluctuation in characteristics due to the season when the fuel is refined, in the case where the fuel with poor volatility is used at the cold engine start-up (for example, 0° C.), the fuel injected from an injector does not sufficiently vaporize, and an actual amount of fuel sucked into a cylinder becomes less than the injection amount of fuel, and further, when the integration constant of the air-fuel ratio feedback control is made larger than the normal value, and the overshoot of the air-fuel ratio occurs, the amount of supply fuel becomes further excessively small. Therefore, there has also been a problem that the misfire is more liable to occur, and the engine stall becomes liable to occur.
As stated above, in the conventional air-fuel ratio feedback control, there have been problems that especially immediately after the cold engine start-up, the air-fuel ratio is not immediately stabilized, the hunting or overshoot occurs in the temporal change of the air-fuel ratio, and the convergence to the target air-fuel ratio becomes slow, and further, the engine is stalled (stopped) in some cases.
Besides, there has been a problem that it is impossible to sufficiently deal with the fluctuation in the characteristics of commercially available fuel among companies, and the fluctuation in the characteristics due to the refining season.